It's the highest percentage recorded by Gallup in 10 years, driven by concern among Republicans -- nearly a quarter of whom see it as the most important problem. Only 9 percent of Democrats and 15 percent of independents view terrorism as so problematic.
Terrorism deposed concerns about the economy, which had consistently been at or near the most commonly cited problems, alongside general worries about the ability of the government to solve the nation's problems.
Immigration and unemployment also slipped as terrorism concerns rose. Overall, the number of people citing any type of economic problem dropped to levels not seen since before the recession.
Last week, Democratic presidential contender Bernie Sanders said that a shift in his campaign's focus from income inequality to terrorism, which he refused to make, would be a "disservice" to working people. Gallup's new survey reinforces that this is risky; Americans are far more worried about terrorism than income inequality.
Other issues important to Democratic voters also paled next to terrorism in the most recent survey, although guns and gun control also saw an uptick in mentions -- almost certainly as a result of the renewed attention to gun availability after San Bernardino. (The spike in October came shortly after the shooting in Oregon.)
Despite the increased concern about terror, favorability of President Obama remains flat -- as it has for years. Democrats love him, Republicans hate him, and independents waver.
In other words, even as the most important problem seen by Americans changes, their view of the person responsible for fixing those problems doesn't.